The present invention pertains to the field of heart failure in devices and methods for treatment thereof.
The syndrome of heart failure is a common course for the progression of many forms of heart disease. Heart failure may be considered to be the condition in which an abnormality of cardiac function is responsible for the inability of the heart to pump blood at a rate commensurate with the requirements of the metabolizing tissues, or can do so only at an abnormally elevated filling pressure. There are many specific disease processes that can lead to heart failure with a resulting difference in pathophysiology of the failing heart, such as the dilatation of the left ventricular chamber. Etiologies that can lead to this form of failure include idiopathic cardiomyopathy, viral cardiomyopathy, and ischemic cardiomyopathy.
The process of ventricular dilatation is generally the result of chronic volume overload or specific damage to the myocardium. In a normal heart that is exposed to long term increased cardiac output requirements, for example, that of an athlete, there is an adaptive process of ventricular dilation and myocyte hypertrophy. In this way, the heart fully compensates for the increased cardiac output requirements. With damage to the myocardium or chronic volume overload, however, there are increased requirements put on the contracting myocardium to such a level that this compensated state is never achieved and the heart continues to dilate.
The basic problem with a large dilated left ventricle is that there is a significant increase in wall tension and/or stress both during diastolic filling and during systolic contraction. In a normal heart, the adaptation of muscle hypertrophy (thickening) and ventricular dilatation maintain a fairly constant wall tension for systolic contraction. However, in a failing heart, the ongoing dilatation is greater than the hypertrophy and the result is a rising wall tension requirement for systolic contraction. This is felt to be an ongoing insult to the muscle myocyte resulting in further muscle damage. The increase in wall stress is also true for diastolic filling. Additionally, because of the lack of cardiac output, there is generally a rise in ventricular filling pressure from several physiologic mechanisms. Moreover, in diastole there is both a diameter increase and a pressure increase over normal, both contributing to higher wall stress levels. The increase in diastolic wall stress is felt to be the primary contributor to ongoing dilatation of the chamber.
Prior art treatments for heart failure fall into four general categories. The first being pharmacological, for example, diuretics. The second being assist systems, for example, pumps. Third, surgical treatments have been experimented with, which are described in more detail below. Finally, multi-site pacing contract the heart muscles at the same time.
With respect to pharmacological treatments, diuretics have been used to reduce the workload of the heart by reducing blood volume and preload. Clinically, preload is defined in several ways including left ventricular end diastolic pressure (LVEDP), or left ventricular end diastolic volume (LVEDV). Physiologically, the preferred definition is the length of stretch of the sarcomere at end diastole. Diuretics reduce extra cellular fluid which builds in congestive heart failure patients increasing preload conditions. Nitrates, arteriolar vasodilators, angiotensin converting enzyme inhibitors have been used to treat heart failure through the reduction of cardiac workload through the reduction of afterload. Afterload may be defined as the tension or stress required in the wall of the ventricle during ejection. Inotropes such as digoxin are cardiac glycosides and function to increase cardiac output by increasing the force and speed of cardiac muscle contraction. These drug therapies offer some beneficial effects but do not stop the progression of the disease.
Assist devices include, for example, mechanical pumps. Mechanical pumps reduce the load on the heart by performing all or part of the pumping function normally done by the heart. Currently, mechanical pumps are used to sustain the patient while a donor heart for transplantation becomes available for the patient. There are at least three surgical procedures for treatment of heart failure: 1) heart transplant; 2) dynamic cardiomyoplasty; and 3) the Batista partial left ventriculectomy. Heart transplantation has serious limitations including restricted availability of organs and adverse effects of immunosuppressive therapies required following heart transplantation. Cardiomyoplasty includes wrapping the heart with skeletal muscle and electrically stimulating the muscle to contract synchronously with the heart in order to help the pumping function of the heart. The Batista partial left ventriculectomy includes surgically remodeling the left ventricle by removing a segment of the muscular wall. This procedure reduces the diameter of the dilated heart, which in turn reduces the loading of the heart. However, this extremely invasive procedure reduces muscle mass of the heart.
The present invention pertains to a device and method for reducing mechanical heart wall muscle stress. Heart wall muscle stress is a stimulus for the initiation and progressive enlargement of the left ventricle in heart failure. Reduction in heart wall stress with the devices and methods disclosed herein is anticipated to substantially slow, stop or reverse the heart failure process, some or reverse the heart failure process, improve contractile function with decrease in isovolumetric contractions and improved isotonic shortening. Although the primary focus of the discussion of the devices and methods of the present invention herein relates to heart failure and the left ventricle, these devices and methods could be used to reduce stress in the heart""s other chambers.
The devices and methods of the present invention are primarily external devices which need not necessarily penetrate the heart wall or transect a heart chamber. These devices can be used instead of, or in addition to, internal or transventricular devices. Unlike transventricular devices, however, avoidance of internal ventricular structures such as valves or chordae is not a concern. It is desirable to limit the size of the external devices to limit inflammatory response that may be created by implanting the device. Additionally, the weight of the device should be limited to reduced movement and forces which can induce inflammatory response or other negative physiologic responses as well. To limit the weight and size of the device, the devices can be constructed with materials with high strength to weight ratios and high stiffness to weight ratios. Size and weight interact to effect the stability of the device on the heart. The devices are preferably stabilized on the heart by tissue ingrowth, sutures, friction fit or the like.
The devices and methods of the present invention can reduce heart wall stress throughout the cardiac cycle including end diastole and end systole. Alternately they can be used to reduce wall stress during the portions of the cardiac cycle not including end systole. Those devices which operate throughout the cardiac cycle can be referred to as xe2x80x9cfull cyclexe2x80x9d devices whereas those that do not operate to reduce wall stress during end stage systole can be referred to as xe2x80x9crestrictivexe2x80x9d devices.